Many microelectronic structures are made with rows of contacts interconnected with one another by leads such as wire bonds. For example, a microelectronic element such as a semiconductor chip may be made with one or more rows of contacts exposed at a front surface of the chip. Each row of contacts extends in a direction referred to herein as the “row direction.” A package substrate may overlie the front surface of the chip. The package substrate may incorporate rows of contacts, commonly referred to as “bond pads” corresponding to the rows of contacts on the chip. Typically, each row of contacts on the package substrate extends parallel to a row of contacts on the chip. The package substrate may include other electrically conductive elements such as terminals for connecting the substrate to an external element, such as a circuit board, and traces connecting the terminals to the contacts or bond pads. The contacts or bond pads of the package substrate are connected to contacts of the chip by leads such as wire bonds. For example, the package substrate may incorporate an elongated slot and one or more rows of contacts extending in a row direction along the slot. The package substrate may be placed on the front surface of the chip so that the slot is aligned with a row of contacts on the chip. The wire bonds extend from the contacts of the package substrate into the slot to the contacts of the chip.
In other cases, a package substrate may have a row of contacts disposed along an edge of the substrate, so that the leads extend from the contacts of the package substrate, over the edge of the package substrate, and downwardly to the contacts of the chip. In still other arrangements, the chip is disposed with a rear surface facing toward the package substrate and with the front, contact-bearing surface facing upwardly away from the package substrate. In this case, the leads may extend over an edge of the chip to the contacts on the package substrate.
Inductances in the leads can pose significant issues. For example, where the contacts of the chip are disposed at small intervals at the row direction the leads are also disposed at relatively small intervals in the row direction. Because the leads lie relatively close to one another, there can be substantial mutual inductance between adjacent leads. Where adjacent leads carry different currents, potentials or signals, there can be cross-talk between the leads. For example, where one lead is carrying a rapidly changing current, the mutual inductance with the adjacent lead will cause a rapidly changing voltage to appear on the adjacent lead. This rapidly changing voltage effectively constitutes “noise” or unwanted voltage fluctuations on the adjacent lead and impairs signal transmissions on the adjacent lead.
Moreover, the mutual inductance between the leads increases the effective inductance along each lead, i.e., the inductance appearing between the contact of the chip and the contact of the package substrate which are connected to one another by a particular lead. This increased effective inductance reduces the speed with which signals can be conveyed along the leads. These problems have become more significant as the speed of operation of components such as semiconductor chips has increased, and the frequency of signals sent along the leads has also increased.
Certain aspects of the present invention alleviate or mitigate these problems.